1. Field of the Invention
The present invention relates to an optical fiber amplifier using hybrid pumping light beams, which provides a rare-earth-doped fiber with hybrid pumping light beams to amplify an optical signal, and more particularly relates to a feedback-type optical fiber amplifier using hybrid pumping light beams, which can reduce the amount of active fiber which constructs the optical fiber amplifier, and improve the amplification efficiency, by looping the hybrid pumping light beams back to the rare-earth-doped fiber by a feedback loop.
2. Discussion of Related Art
Optical communication techniques transmitting information through optical fibers have been developed and are widely being used. The optical communication techniques which can transmit large amount of information at high speed, are applied for information communications between countries through submarine cables because they do not suffer from signal disturbance or crosstalk due to electromagnetic induction. As multiplex or network techniques for the optical communications have been developed so far, the optical communication techniques gradually enlarge the range of their application to the key communication networks for high-speed broadband multimedia communications including voice and data communications between switches, cable TV or video on demand (VOD).
The optical communication techniques have been improved according to the development of optical signal amplifiers, which provide high-speed optical signal transmission and superlong-distance transmission. Recently, there have been actively carried out researches about amplifiers having flat gain wavelength, which is used in wavelength multiplex, and high-gain amplifiers for image distribution techniques.
An early optical signal amplifier converts an optical signal into an electric signal through an avalanche-type photodiode to amplify, and reconverts the amplified electric signal into the optical signal using a laser diode. Present optical signal amplifiers employ rare-earth-doped fibers so that the signal conversion process for optical signal amplification can be omitted. The aforementioned rare-earth-doped fiber is formed in a manner that an active optical fiber is doped with a rare earth ion such as Er, Pr and Nd. When a pump light beam having a predetermined wavelength is supplied to the rare-earth-doped fiber, stimulated photon having a predetermined wavelength is emitted due to excitation of the rare earth ion, which amplifies the optical signal propagated through corresponding optical fiber ultimately.
FIG. 1 shows a configuration of a conventional prior art optical fiber amplifier using the rare-earth-doped fiber. Referring to FIG. 1, an optical signal S is coupled with a first optical line 1, and pumping light beam P is coupled with a second optical line 2, first and second optical lines 1 and 2 being coupled to the input of a multiplexer 3. A third optical line 4 corresponding to the output of multiplexer 3 is coupled to a fourth optical line 7 which is the output line through the rare-earth-doped fiber 5 and isolator 6. In this configuration, optical signal S and pumping light bream P applied through first and second optical lines 1 and 2 respectively are coupled with each other by multiplexer 3 so that they are included together in third optical line 4 corresponding to the output of multiplexer 3.
Optical signal S and pumping light beam P are applied to the rare-earth-doped fiber 5 where pumping light beam P excites rare earth ions doped thereinto, to generate stimulated photon having a predetermined wavelength. This light is introduced into optical signal S and effects optical amplification. Isolator 6 prevents opposite optical signals from being introduced into the rare-earth-doped fiber 5, which proceed in a direction opposite to the optical signal S and include, for example, pumping light beam from another rare-earth-doped fiber located in the following stage or reflection signal of the optical signal S. In the above-described optical fiber amplifier, an optical beam having a wavelength of 1520 to 1570 nm is used as optical signal S, and optical beam having a wavelength of 1480 or 980 nm is used as pumping light beam P. Here, the pumping light beam with a wavelength of 1480 nm is used to generate maximum gain, and pumping light beam with a wavelength of 980 nm is used to provide low noise characteristic. Accordingly, both 1480 nm and 980 nm pumping light beams are widely used in various optical transmission systems.
FIG. 2 shows a configuration of a conventional prior art optical fiber amplifier using the aforementioned hybrid pumping light beams. Referring to FIG. 2, optical signal S is coupled with a first optical line 21, and a first pumping light beam P1 having a predetermined wavelength, for example, 1480 nm, is coupled with a second optical line 22. First and second optical lines 21 and 22 are coupled to the input of a first wavelength division multiplexer 23 which couples optical signal S and first pumping light beam P1 to a third optical line 24 connected to an input of a second wavelength division multiplexer 26. A second pumping light beam P2 having a predetermined wavelength, for example, 980 nm, is coupled with a fourth optical line 25 which is connected to the other input of second multiplexer 26. Second multiplexer 26 couples optical signal S and first pumping light beam P1 supplied through third optical line 24 and second pumping light beam P2 sent through fourth optical line 25, to output them through a fifth optical line 27 which is connected to the input of rare-earth-doped fiber 28. The output light beam of rare-earth-doped fiber 28 is coupled with a sixth optical line 30 corresponding to an output line through an isolator 29.
In this configuration, optical signal S and first pumping light beam P1 are coupled by first multiplexer 23, and the output of first multiplexer 23 and second pumping light beam P2 are coupled in second multiplexer 26. Then, the coupled optical signal S, first and second pumping light beams P1 and P2 are supplied from second multiplexer 26 to rare-earth-doped fiber 28. First and second pumping light beams P1 and P2 excite rare earth ions of rare-earth-doped fiber 28, to generate stimulated photon which is introduced into optical signal S to be amplified.
However, this optical fiber amplifier has the following problems. An optimum length of rare-earth-doped fiber 28 is generally set according to the wavelength of pumping light used as exciting light. For example, when the wavelength of pumping light is 1480 nm, the optimum length is set to approximately 15 m, and, when 980 nm, it is set to approximately 9 m. When the actual length of the rare-earth-doped fiber is shorter than the optimum length, its amplification efficiency is deteriorated. On the other hand, if longer, optical signal attenuation occurs. With the above-described conventional optical fiber amplifier configuration, since the first and second pumping light beams coupled by the multiplexer are supplied to the rare-earth-doped fiber, it is impossible to adequately set the length of the rare-earth-doped fiber according to them.
Meanwhile, U.S. Pat. No. 5,185,826 has been proposed an optical fiber amplifier which effectively amplifies optical signals using hybrid pumping light beams, which is shown in FIG. 3. Referring to FIG. 3, an optical signal S and first pumping light beam P1 with a first wavelength are coupled by a first multiplexer 31 whose output light beams P1 and S are supplied to a first rare-earth-doped fiber 32. Optical signal S has a wavelength of 1520 to 1570 nm, first pumping light beam P1 has, for example, a wavelength of 1480 nm, and first rare-earth-doped fiber 32 is constituted of, for example, active optical fibers doped with Er.
The output of first rare-earth-doped fiber 32 is coupled to an input of a second multiplexer 33, and second pumping light beam P2 having a wavelength of 980 nm is coupled to its other input. Second multiplexer 33 couples light beams P1 and S from rare-earth-doped fiber 32 and second pumping light beam P2, and supplies them to a second rare-earth-doped fiber 34 whose output is coupled to an output optical line 9 through an isolator 35. In this configuration, when the length of first rare-earth-doped fiber is L1, and the length of second rare-earth-doped fiber is L2, the length of L1+L2 is set as an optimum length for first pumping light beam P1, and length L2 is set as an optimum length for second pumping light beam P2.
First pumping light beam P1 coupled through first multiplexer 31 is applied to first rare-earth-doped fiber 32 to excite the rare earth ion doped thereinto, thus effecting optical amplification. Here, since the length of first rare-earth-doped fiber 32 is set shorter than an optimum length for first pumping light beam P1, first pumping light beam P1 applied to rare-earth-doped fiber 32 is not all lost in first rare-earth-doped fiber 32 but some of it remains. The residual first pumping light beam P1 outputted from first rare-earth-doped fiber 32 is coupled with second pumping light beam P2 by second multiplexer 33, and they are applied to second rare-earth-doped fiber 34 where first and second pumping light beams P1 and P2 carry out amplification operation. That is, since first pumping light beam P1 executes amplification in both first and second rare-earth-doped fibers 31 and 34, the length of rare-earth-doped fiber for first pumping light beam P1 is set to L1+L2. And the length of rare-earth-doped fiber for second pumping light beam P2 is set to L2 because second pumping light beam P2 performs amplification operation only in second rare-earth-doped fiber 34. Accordingly, the effective optical fiber amplifier can be realized using hybrid pumping light beams.
However, the rare-earth-doped fiber used in the above optical fiber amplifier is fabricated through complicated processes and highly expensive. This requires that the length of rare-earth-doped fiber is reduced. Furthermore, in the aforementioned optical fiber amplifier, the pumping light beam used as the exciting light of the rare-earth-doped fiber corresponds to noise signal in terms of the optical signal transmitted through corresponding optical fiber. Accordingly, to prevent the residual pumping light in the rare-earth-doped fiber from being transmitted through the optical fiber, the conventional optical fiber amplifier has a reflection mirror at its output terminal, to reflect the pumping light beam. However, the reflection mirror reflects not only the pumping light beam outputted from the rare-earth-doped fiber but also a portion of the optical signal transmitted through the optical fiber. Thus, it may deteriorate the output level of the optical signal.